1. Field of the Invention
This invention relates generally to a method and apparatus for recording and/or reproducing data on a disk formatted in accordance with a constant linear velocity (CLV) system, and more particularly is directed to such method and apparatus suitable for use with a write once optical disk drive.
2. Description of the Prior Art
It is known to employ disks formatted in accordance with constant angular velocity (CAV), constant linear velocity (CLV) and modified constant angular velocity (MCAV) systems for the recording and reproducing of data in sectors arranged on the disk while suitably controlling the speed of rotation of the disk by a spindle motor, for example, as shown in detail in U.S. Pat. No. 4,660,189, which has a common assignee herewith.
As generally illustrated in FIG. 4, in such known apparatus for controlling the rotation of a disk 1 coupled to a spindle motor 2 through a rotary shaft 3 while a recording and reproducing head (not shown) is moved in a radial direction R relative to the disk 1, the rotary shaft 3 of the motor 2 is further connected with a rotation sensor 4, such as, a frequency generator or the like, that provides a pulsed signal with a frequency corresponding to the rotational speed of the disk 1. Such pulsed signal is applied to a rotational speed detector circuit 5 which, in response thereto, provides a rotational speed signal S1 having a value or level corresponding to the rotational speed of the disk. The rotational speed signal S1 is supplied to one input terminal of a comparator circuit 6 which, at another input terminal, receives a predetermined reference signal S2 from a terminal 7. The comparator circuit 6 compares the received rotational speed signal S1 with the reference signal S2 and provides a corresponding difference signal S3 which is supplied to a motor drive circuit 8. The motor drive circuit 8 drives the motor 2 in response to the difference signal S3 so as to vary the rotational speed of the motor 2, and hence of the disk 1, in the direction for reducing the difference signal S3 to 0, that is, in the direction for equating the rotational speed signal S1 with the reference speed signal S2. Thus, the control circuit illustrated in FIG. 4 constitutes a so-called servo loop. Although, in practice, such servo loop desirably includes a phase control loop and a velocity control loop as well as a phase compensating circuit for increasing the servo accuracy, such additional circuits are not shown in FIG. 4 for the sake of simplicity.
When recording and reproducing signals on the disk 1 in accordance with the CAV system, the rotational speed of the disk 1 is maintained constant, that is, the reference signal S2 applied to the comparator circuit 6 has a constant value, and, as a result thereof, a relatively short access time is attainable even when moving the head a relatively large radial distance on the disk. As shown in FIG. 5A which represents the format of the disk 1 when recorded according to the CAV system, the hatched areas 10 and 11 needed to record unit sectors in respective tracks adjacent the inner and outer peripheries, respectively, of the recordable area of the disk have different peripheral lengths. More specifically, the length in the peripheral direction along each track that is required to record a unit sector is progressively increased from track to track from the inner periphery to the outer periphery of the area of the disk available for recording of data. By reason of the foregoing, when using the CAV system, the recording density is progressively lowered from track to track in the radial direction from the inner periphery to the outer periphery of the recordable area of the disk 1 so that, when considered in the aggregate, the recording capacity is relatively small.
In the case of the CLV system, the rotational speed at which the disk 1 is rotated during recording and reproducing data thereon is changed from track to track in inverse proportion to the radius of the track on the disk then being scanned for the recording or reproducing of data thereon. As a result of the foregoing, when the disk 1 is recorded according to the CLV system, the format thereof illustrated on FIG. 5B is characterized by areas of equal peripheral lengths required for the recording of unit sectors in tracks adjacent the inner and outer peripheral areas of the recordable area of the disk, as indicated by the shaded or hatched areas 12 and 13, respectively. Accordingly, the same recording density is obtained on each of the tracks comprising the recordable area of the disk with the result that a theoretically maximum recording capacity is achieved. Such recording capacity attainable through the use of the CLV system is about 1.5 times the recording capacity attained when using the CAV system.
However, when using the CLV system, in order to maintain the linear velocity at a constant value, the rotational speed or angular velocity of the spindle motor 2 must be changed in accordance with changes in the position of the recording and reproducing head in the radial direction R of the disk 1. In other words, the value of the reference signal S2 supplied to the comparator circuit 6 (FIG. 4) must be varied as a function of the number of the track being scanned by the recording and reproducing head, which number is counted in the radially outward direction. Thus, when using the CLV system, the rotational speed of the disk must be changed in accordance with access movements of the recording and reproducing head. The need to change the rotational speed of the disk with changes in the position of the track being scanned by the recording and reproducing head requires a substantial increase in the access time when the recording and reproducing head is moved a relatively large radial distance, for example, from a track near the inner periphery to a track near the outer periphery of the recordable area of the disk. The increased access time is required to permit the necessary substantial change in the rotational speed of the disk which is opposed by the inertial tendency of the disk to continue rotation at its original speed.
In both the CAV system and the CLV system, the number of sectors recorded and reproduced per unit time is the same when the head scans any of the tracks within the recordable area of the disk. Therefore, in the case of the CAV system and in the case of the CLV system, the frequency of the clock employed for the recording and reproducing operation performed anywhere within the recordable area of the disk is unchanged.
The MCAV system seeks to effectively utilize the advantages of the CAV system, that is, the relatively short access time of the CAV system, while increasing the recording capacity relative to the latter. As shown in FIG. 5C which illustrates the format of a disk 1 recorded by the MCAV system, it will be seen that, in such case, the recordable area of the disk is divided into a plurality of physically predetermined zones 16a, 16b, 16c, arranged successively in the radially outward direction and each comprised of a respective plurality of circular tracks, and the recording and reproducing is carried out in accordance with the CAV system in each of the zones 16a, 16b, 16c. When the recording and reproducing head moves from one zone to the next, the frequency of the recording and reproducing clock is changed to the maximum clock frequency that can be utilized within the newly encountered zone for effecting the recording and reproducing operations therein. As a result of the foregoing, the hatched areas 17a, 17b and 17c required for the recording of unit sectors in the radially innermost tracks of the zones 16a, 16b, 16c, respectively, are of the same lengths. Therefore, the recording density is the same in each of the tracks, and the number of sectors recorded in each track is increased from zone to zone in the radially outward direction. Since the frequency of the recording and reproducing clock is changed from zone to zone in the MCAV system, the data transfer rate is similarly changed from zone to zone in such system.
Referring now to FIG. 6 for a graphic comparison of the recording capacities of disks recorded according to the CAV, CLV and MCAV systems, respectively, it will be seen that the abscissa represents the radius of the disk from the radius r.sub.0 of the track at the inner periphery of the recordable area of the disk to the radius r.sub.max of the track at the outermost periphery of the recordable area, while the ordinate represents the number of sectors in each of the tracks having a radius between r.sub.0 and r.sub.max. The horizontal dashed line on FIG. 6 represents the number of sectors in each of the tracks between the radii r.sub.o and the r.sub.max on a disk recorded according to the CAV system; the upwardly inclined solid line represents the numbers of sectors in the tracks between the radii r.sub.o and the r.sub.max on a disk recorded according to the CLV system; and the upwardly stepped dot-dash line represents the numbers of sectors in the tracks between the radii r.sub.o and r.sub.max on a disk recorded according to the MCAV system. Therefore, the rectangular area under the dashed horizontal line on FIG. 6 represents the recording capacity of a disk recorded according to the CAV system, the trapezoidal area under the inclined solid line on FIG. 6 represents the recording capacity of a disk recorded according to the CLV system, and the area under the stepped dot-dash line on FIG. 6 represents the recording capacity of a disk recorded according to the MCAV systems. From the foregoing, it is apparent that the CLV system affords the maximum recording capacity which is theoretically 1.5 times the recording capacity attainable by the CAV system and only slightly larger than the recording capacity attainable by the MCAV system.
Although the CAV, CLV and MCAV systems have certain respective desirable characteristics, each of these known systems is burdened with a respective disadvantage. Thus, for example, although the CAV system has a short access time, as described above, the recording density measured in the peripheral direction along each track is decreased from track to track in the radially outer direction with the result that the overall recording capacity is relatively small.
Although the CLV system provides a relatively large recording capacity which, as noted above, is theoretically 1.5 times the recording capacity attainable with the CAV system, the rotational speed of the disk being recorded or reproduced must be changed from track to track, that is, in accordance with the radial position of the recording and reproducing head, so as to achieve the desired constant linear velocity and, as a result of the need to effect such changes in the rotational speed, the access time is undesirably increased.
Although the MCAV system can obtain a recording capacity substantially equal to that of the CLV system while employing a constant rotational speed so that the access time is desirably short, the frequency of the recording and reproducing clock is changed as the head moves radially from one to another of the physically predetermined zones into which the recordable area of the disk is divided so that it is necessary to constantly effect accurate control of the absolute address or radial position of the recording and reproducing head. Furthermore, since the data transfer rate is changed upon each change in the frequency of the recording and reproducing clock at the inner and outer peripheries of each zone while the rotational speed of the disk remains constant, effecting the desired control becomes complicated.